Electrochemical treatment of lignins

ABSTRACT

Lignin is cleaved electrolytically into smaller molecules than the starting lignin by passage of electric current through an aqueous alkaline solution of lignin at a temperature above 100° C. while mixing. The yield is greater than 6%.

This invention relates to an electrochemical process for theelectrochemical oxidative degradation of lignins and related substances,and to an electro-chemical cell in which the process may be performed.

Lignin is, after cellulose, the principal constituent of the woodystructure of higher plants. About 25% of dry wood consists of lignin, inpart deposited in the xylem cell walls and in part located in theintercellular spaces, where it may constitute as much as 70% of thesolid materials present.

The exact chemical structure of lignin, either in wood, where it isusually bonded to plant polysaccharides, or when separated from otherwood substances, is not fully known. Much is known however about thestructure of certain isolated lignines. For example the lignin isolatedfrom coniferous trees is though to be a polymer resulting fromenzymically induced oxidation of coniferyl alcohol.

Lignins appear to be constructed of phenylpropane units, substitutedprinciaplly by methoxy and hydroxy groups, and joined in a polymericstructure by various types of linking groups.

The most common types of substituted phenylpropane units in bothconiferous and deciduous lignins are hydroxyphenylpropane (i),syringylpropane (ii) and guaiacylpropane (iii) units: ##STR1##

The relative proportions of these three units vary between coniferousand deciduous lignins, eg coniferous lignin contains about 14% (i), 7%(ii) and 79% (iii), whereas deciduous lignins contain about 3 of (ii) to2 of (iii). As well as the methoxy and hydroxy groups, smallerquantities of other minor functional groups may also be present on theseunits.

The phenylpropane units in lignin are linked mainly by carbon-carbonbonds and by ether linkages. Spectroscopic data suggest that about 25%of the units are linked as biphenyl linkages. The phenolic oxygen inabout 66% of the units is present as an ether linkage.

Some examples of typical linkages are shown below together with theapproximate percentages to which they occur in a typical ligninstructure. ##STR2##

But a wide variety of other linkages probably also exist in lignins,particularly between the propyl chains to form cyclic species such ascyclic ethers, such as ix and x below: ##STR3##

By means of such linkages the phenylpropane units are linked into alarge polymeric structure, probably randomly linked. Average molecularweights for coniferous lignin is over 10,000, whilst the averagemolecular weight of deciduous lignin probably does not exceed 5000.

A suggested structure for coniferous lignin incorporating such bondingis shown in Kirk-Othmer `Encyclopaedia of Chemical Technology` 2nd Edn,Vol 12 (1967) p 367.

Millions of tons of lignins are potentially available annually fromindustry, such as wood and bark wastes from the lumber industry, thematch industry, and particularly from the wood pulp and paperindustries.

In the pulp industry lignin is usually obtained as dissolvedlignosulphonic acid or as lignosulphonate salts as a result of cookingwood chips under pressure in the presence of aqueous sulphurous acid orsulphites, which leaves the cellulose as a residue for example for papermaking. From the solution the acid or salt may be obtained by drying.

From these lignosulphonates, alkali lignate salts may be prepared byhydrolysis using aqueous hydroxides, especially sodium and calciumhydroxides. Alkali lignates may also be prepared directly from woodchips by cooking them with sodium hydroxide, optionally with a littlesodium sulphide present. These lignates are almost free from non-ligninorganic constituents but may contain a little combined sulphur if theyhave been prepared from the sulphonates or if sodium sulphide has beenused.

Another source of lignin which is likely to become of increasingimportance is straw. Millions of tons of straw are wasted each year, egby burning. Straw contains about 16% of lignin. Although straw lignin isbuilt up of the units discussed above, it has a slightly differentstructure to wood lignin. Straw lignin may be extracted chemically eg bysodium hydroxide or sodium sulphite treatment, in much the same way aswood lignin.

Lignin may also be extracted from plants eg wood and straw by treatmentof the plant in a suitable form such as woodchips, with phenol at atemperature of around 110° C. These conditions hydrolyse hemicellulosesand leave the lignin in a conveniently solublized form known as"organosolv lignin" which is commercially available. Organosolv ligningenerally has a molecular weight of around 2000 to 5000, and has alignin structure as discussed above but with some of the methoxy ringsubstituents removed. Another commercial process used hydrogen fluorideto extract lignin from plants, in a form known as "HF lignin".

As is well known, under pressure and temperature, over a geologicalperiod of time, plants are gradually converted into coal, with acorresponding gradual change of chemical structure, including thegradual dissappearance of lignin. In certain coals, including peats,soft brown coals, dull brown coals, bright brown coals, bituminous hardcoals and sometimes even anthracites, lignin will be present, but inever decreasing amounts. Lignin may be extracted from coals whichcontain it by methods similar to those described above, with varyingdegrees of success, and for the purposes of this description the term"lignite" or "lignitic coal" will be used for coals from which ligninmay be extracted.

The term "lignin" used herein, unless otherwise stated, refers to allforms of lignin.

Lignin and its derivatives such as sulphonate are very useful in anumber of industries such as in leather tanning and concrete (asdispersants), in which they are used directly. Lignin may also bechemically degraded, for example by thermal degradation, alkalinefusion, pressure hydrogenation and oxidation to yield valuable organicchemicals, especially the flavouring agent vanillin,(4-hydroxy-3-methoxybenzaldehyde) (xi). ##STR4##

The most widely used methods for oxidation of lignin use nitrobenzene,metal oxides such as of copper, mercury, silver and cobalt, molecularoxygen in alkaline solution, peracetic acid or acidic hydrogen peroxide,sodium hypochlorite, chlorine dioxide or sodium chlorite as oxidisingagents. To a lesser extent dichromates, permanganates and ozone havebeen used.

The use of each of the above oxidising agents presents problems.Nitrobenzene is expensive and is itself oxidised to highly undesirable(eg in the food industry) by-products including aniline, azobenzene and4-hydroxy azobenzene among others. As well as their toxicity, thepresence of these organic by-products adds to the difficulty ofseparation of the desired products. Metal oxides are also expensive, maybe toxic, are difficult to recover and often oxidise the products oflignin degradation further. Oxygen must be used at elevated temperaturesand temperatures which are potentially hazardous and may causeoveroxidation. Peracetic acid and hydrogen peroxide are expensive andcause overoxidation eg to carboxylic acids. The chlorine based oxidantsare corrosive and dangerous (ClO₂ is explosive) and give unstableproducts which are difficult to characterise. Dichromates, permanganatesand ozone cause degradation of the aromatic nucleus of lignins to lowermolecular weight products of less value.

There has been some work on electrochemical oxidation of lignins (Refs 1to 4) at temperatures around ambient and below 80° C., but the resultswere discouraging and appeared to achieve little more than modifying thelignin molecule by cleavage of the side chain to increase the --OH andCO₂ H content. The reported yields of useful low molecular weightproducts such as vanillin and vanillic acid were very low, eg ca 2-3%,which could be attributed to alkaline pre-treatment causing cleavage,and subsequent oxidation of the small phenolic fragments to aldehydesand acids.

The same workers, using Ni, Ni peroxide and glassy carbon, found thatanodic oxidation of lignin in an alkaline medium gave no significantcleavage of the lignin at ambient temperature, and an increase to astill relatively useless 2-6% cleavage at 110° C. Over such atemperature change it would be expected that a considerable increase inyield would be obtained.

Further discouragement is found in the tendency for anticipated monomersto form multicomponent mixtures of polymeric products even at roomtemperatures.

It is an object of the present invention to provide a method ofoxidative degradation of lignin which avoids the disadvantages of theprior art processes and which provides advantageous conditions ofelectrochemical oxidation. Other objects and advantages will becomeapparent from the following description.

According to the present invention, a process for the electrolyticcleavage of lignin at a yield greater than 6% comprises passing anelectric current through an aqueous alkaline soluton of the lignin at atemperature above 100° C. whilst maintaining mixing of the solution.Yields of 10% or more may be achieved by the process.

Using the process of the invention under the conditions discussed belowefficient electrolytic cleavage of the lignin occurs, and this cleavagemay be complete ie to provide useful compounds including monocycliccompounds such as vanillin (xi), or partial, so as to produce dimers,trimers or higher oligomers of monocyclic species which may also beuseful.

The process of the invention is normally carried out in anelectrochemical cell provided with electrodes between which the electriccurrent is passed and which is adapted to withstand the corrosiveeffects of the hot alkali solution, the temperature and consequentpressure. Suitable cell designs will be apparent to those skilled in theart, and the inventors have found that a stainless steel cell, linedwith Teflon (trade mark), is suitable. The cell should be sealed toavoid boiling of the water and should be fitted with a safety valve incase of overpressure. The above layout is entirely conventional.

On an industrial scale, the process may be carried out in electrolyticcells of conventional design, eg flow cells, and the construction ofcells to withstand the conditions of the process would present noproblem whatever to a chemical engineer skilled in the art. Theprinciples discussed herein with respect to laboratory or pilot scalecells are entirely applicable with adjustment to scale to an industrialplant.

A preferred alkali is sodium hydroxide, but other alkali metalhydroxides could also be used, a preferred concentration being 2.5-3.5M.Lower concentrations may be used, but the efficiency of the processreaches a plateau at this concentration and no advantage is usuallygained by the use of more concentrated alkali.

The lignin may be made up into the aqueous alkali either by using thelignin itself, or by using a compound of lignin which is capable ofbeing hydrolysed under the alkaline conditions of the solution, eitherat ambient or eleva ted temperatures, into soluble lignin or into alignate salt. For example a lignin sulphonate or sulphonic acid may beused. It may also be possible to use certain lignites in the process,provided that these are well crushed and the design of the cell is suchthat the presence of solid lignites will not interfere with itsoperation. Similarly it may be possible to use vegetable matter whichcontains lignin eg straw, in the process of the invention without anyprior extraction of the lignin. In this case too the possible problem ofthe solid residue should be noted. Filters in the cell eg in the case ofa flow cell could be used. The lignin present or formed in the alkalinesolution may be converted under the alkaline conditions into a lignatesalt, and therefore these too may be used to make up the solution.Lignins and lignin compounds from coniferous, deciduous and othersources may be used. Some commercially available lignins may beinsoluble in the alkali used, eg HF lignin may be, and this should bechecked beforehand.

The concentration of lignin present in the solution has an upper limitdetermined by solubility and viscosity, as at high concentrations thesolution may become too thick to mix efficiently. Prehydrolysis of thelignin prior to electrolysis may help to solubilise the lignin, reducethe viscosity, and increase the efficiency of oxidation and thus theyield of useful products after electrolysis. Typically in prehydrolysislignin is heated in the presence of an alkali metal hydroxide underconditions similar to those of the subsequent electrolysis ie aqueoussolution above 100° C. A preferred temperature range is 170°-180° C. fora suitable period eg 2-4 hours prior to electrolysis but times andconditions are variable. This prehydrolysis may conveniently beperformed in the electrolytic cell prior to passing the current.Successful electrolytic oxidative cleavage in the process of theinvention was obtained using 1-2 wt% of lignin in the solution. If alignin compound such as a ligninsulphonate is used, which is hydrolysedunder the reaction conditions or prehydrolysed, the amount of such acompound used should not exceed the stoichiometric amount which can behydrolysed by the amount of alkali present.

The efficiency of the process is increased by increasing thetemperature, and a temperature of 170°-190° has been found to be optimumwith no practical advantage in using a higher temperature. Below 100° C.the efficiency of the process is generally too low to be worthwhile.

An importance factor in attaining a high yield of the desired lowmolecular weight cleavage products is the need to mix the solutionduring the course of the process. This may be achieved by anyconventional mixing or stirring mechanism, eg on a small scale by usinga stirrer in the cell, or on an industrial scale by a stirrer or aconventional cycling pump. Means for mixing the solution will beapparent to those skilled in the art.

A direct current is passed between the electrodes of the cell. It ispreferred to use a low current density so that hydrogen and oxygenevolution are minimised for safety reasons (this mixture of gases isexplosive) and to maximise the current efficiency of cleavage byoxidative degradation of the lignins. The cell voltage appears to beless critical than current density, the lowest possible voltage toachieve cleavage of the lignin with the cell design used is generallypreferred. The cell is normally set up and the voltage adjusted toachieve this.

The desirability of a current density as low as possible whilstmaintaining cleavage also influences the electrode design. The anodeshould be of large surface area to achieve this, and may thus forexample be in the form of a gauze. When the anode is a gauze, theoptimum current density is in the range of 0.2-10 mAcm⁻² quoted in termsof the nominal surface area of the gauze. With an anode of othergeometry a similar figure of current density would apply. Above 10mAcm⁻² over oxidation begins to occur leading to the formation ofgaseous products and around 4 mAcm⁻² eg 3-5 mA cm⁻² appears to beoptimum. The electrodes may be made of the variety of conventionallyused electrode materials which are capable of resisting hot alkali. Forthe cathode, among others, nickel, copper, vitreous carbon and lead havebeen found suitable. To minimise hydrogen evolution from the cathode itis preferred to use a cathode material with a high hydrogenoverpotential, and for this reason lead is preferred although nickel ispreferred if the products are for human or animal consumption due to thepossibility of contamination with lead. For the anode, among otherscopper, vitreous carbon and nickel have been found suitable. Nickel hasbeen found to be particularly effective at resisting corrosion and ingiving a good yield of degradation products, and is preferred,especially in the form of a gauze.

Various electrode geometries will be apparent to those skilled in theart with the intention of producing a cell with a low current density atconvenient working voltages and for electrolysing as large a volume ofthe cell contents as possible. A suitable electrode geometry utilises acentral rod anode and a concentric cylindrical cathode, or gauzes in a"Swiss roll" configuration of the anode and cathode such that the gauzesare rolled up together in a cylindrical manner, the two electrodes beingseparated from one another by some insulating means such as Teflon(trade mark) mesh. Other insulating means and electrode geometries (forexample a cylindrical anode surrounding a rod cathode) will be apparentto those skilled in the art, and adoptation to an industrial scale wouldpresent no problem.

The time for which the process is carried out will depend of course uponthe cell dimensions, concentration, temperature etc, and the yield fromthe degradation which is considered viable.

After the process of the invention has been carried out, the degradationproducts may be extracted from the aqueous solution by essentiallyconventional means. For example the hot alkaline solution is cooled toambient temperature, acidified with an acid which does not affect thedesired products, eg hydrochloric acid, extracted with an organicsolvent, eg chloroform, which may then be neutralised, dried andevaporated to yield the product in a conventional way.

The products of the process may include a variety of useful compounds,such as vanillic acid (4-hydroxy-3-methoxybenzoic acid),4-hydroxy-benzaldenhyde, vanillin, 4-hydroxyacetophenone, acetovanillone(4-hydroxy-3-methoxyacetophenone) and others, These compounds may beseparated from the crude yield by processes apparent to the chemist, egon a lab scale by chromatography and on an industrial scale by wellestablished methods. The proportions of the various compounds presentwill depend upon the type of lignin used, and electrolysis conditions.

The process of the invention provides a number of advantages over priorart processes as well as the possibility of fine control of the productdiscussed above. The aqueous alkaline electrolyte is cheap and presentsno undue problems of disposal. No additional undesirable chemicaloxidants need be present, and the problem of isolating these from thereaction mixture, and the possible dangers from their use and avoided.As well as these advantages, the reaction conditions (temperature,pressure, current density) and relatively mild and easily controlled,and the process can be carried out at a large (industrial) scale withreadily available simple equipment as conventionally used in theelectrolysis art. Over previous electrochemical oxidation processes theinvention provides an advantageous set of electrolysis conditions whichattain a very substantially improved yield. Although many of theproducts mentioned above may be obtained from other sources, eg thepetrochemical industry the price of oil is subject to unpredictablefluctuation, and the invention provides a potential alternative.

Although described herein with reference to lignins and compoundsrelated to lignins it is to be expected that the process of theinvention will be applicable to the electrochemical oxidation of a widerange of natural products to yield useful degradation products, such asin particular the oxidation of soluble celluloses to hemicelluloses orof soluble polysaccharides eg sugars to glyoxals and carboxylic acids.

BRIEF DESCRIPTION OF THE DRAWINGS

The process of the invention will now be described by way of exampleonly with reference to the accompanying FIGS. 1 and 2 and 3 which showcutaway views of two electrochemical cell in which the process may becarried out.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1 an electrochemical cell comprises a stainless steelvessel (1) closed with a stainless steel lid (2) held in positionagainst internal pressure by bolts (3) the seal being maintained by `O`rings (4). The interior of the vessel (1) is lined with Teflon (trademark) (5). Through the lid (2) pass a cathode (6) in the form of a leadrod, and an anode connector (7) connected to a nickel gauze anode (8) inthe form of a cylinder completely encircling the cathode (6). Insulationand airtightness where the cathode (6) and anode connector (7) passthrough the lid (2) are maintained by Teflon (trade mark) sleeves (9).The lid (2) is also fitted with a safety valve and means for releasingpressure, shown conventionally (10). Within the vessel (1) is containedan alkaline solution of lignin (11), which is stirred by a magneticstirrer (12) in the form of a cylinder with internal propellor blades,operated by a stirring unit (not shown) outside the cell. In use thevessel (1) and contents (11) are heated to and maintained at theoperating temperature by an external heater (not shown).

Referring to FIGS. 2 and 3, an electrochemical cell comprises astainless steel vessel (13) designed so that is has two main chambers(14) and (15) which are joined together by two ducted pipes (16). Thechambers (14) and (15) are closed with two stainless steel lids (17) and(18) which are held in position against internal pressures by bolts (19)the seal being maintained by `O` rings (20). The chamber (14) of thecell is lined with Teflon (trade mark) (21). Through the lid (17) pass acathode connector (22) and an anode connector (23) which are connectedto a "Swiss roll" arrangement of nickel gauze anode (24) and cathode(24a). The anode and cathode are separated by a Teflon (trade mark) mesh(25a). Insulation and airtightness where the connectors for anode andcathode pass through the lid (17) is maintained by Teflon (trade mark)sleeves (25). The lid (17) is also filled with a safety valve and meansfor releasing pressure shown conventionally (26). Within the vessel (13)is contained an alkaline solution of lignin (27), which is stirred by amagnetic stirrer (28) contained in the chamber (15). In use the vessel(13) and contents (27) are heated to and maintained at the operatingtemperature by an external heater not shown. This type of cellillustrates the possibility of a flow type of cell in which electrolyteis rapidly circulated through the system thus maintaining stirring.

EXAMPLE 1

Organosolv lignin extracted by phenol from spruce (conifer) (0.25 g) wasdissolved in aqueous sodium hydroxide (25 ml, 3M) and introduced intothe cell shown in FIG. 1 prior to sealing. The cell had a capacity of ca35 ml and had a nickel gauze anode of mesh size 40 with a nominalsurface area 18 cm². The cell was heated to 170° C. and electrolysis wascontanued at 70 mA for 4 hours, during which 10³ coulombs was passed.The voltage required was always less than 5 V, usually 1.8-2.0 V. Thecell was then cooled, pressure released, and the contents decanted off.The contents were then acidified to pH2 with hydrochloric acid. The acidmixture was shaken with chloroform (3×70×1) and the chloroform layerseparated off, neutralised with sodium carbonate and dried with sodiumsulphate.

Filtration and evaporation yielded a light brown semi-solid product(0.072 g, 28% yield by weight) using a more efficient stirrer a 36%yield was obtained. Analysis of this product by chromatrographic methodsshowed that the major products were:

    ______________________________________                                        Identification    Relative yield mole %                                       ______________________________________                                        Vanillic acid     19                                                          4-hydroxhbenzaldehyde                                                                           51                                                          vanillin          17                                                          4-hydroxyacetophenone                                                                            9                                                          acetovanillone     4                                                          (plus 2 unidentified products)                                                ______________________________________                                    

The experiment was repeated using other anode and cathode materials.This was found to affect the yield, all other conditions being equal, asbelow:

    ______________________________________                                        Anode         Cathode  Yield (%)                                              ______________________________________                                        Copper        Copper   15-20                                                  Nickel        Copper   17-20                                                  Nickel        Lead     20-36                                                  ______________________________________                                    

EXAMPLE 2

Phenol extracted spruce lignin (obtained from Battelle) (0.30 g) wasdissolved in aqueous sodium hydroxide (60 ml, 3M) and introduced intothe cell, shown in FIG. 2, prior to sealing. The cell had a capacity ofabout 80 ml and had a nickel gauze anode of mesh size 40 with a nominalsurface area of about 100 cm². The cathode made of lead and anode werearranged in the above mentioned Swiss roll configuration with Teflon(trade mesh) to separate them. The cell was heated to 170° C. andelectrolysis was carried out at 300 mA for 3 hours during which time3×10³ coulombs was passed. The voltage required was always less than 5V, usually 1.8-2.0 V. The cell was then cooled, pressure released andthe contents decanted off. The resulting solution was then acidified topH2 with hydrochloric acid. The acidic mixture was shaken withchloroform (3×70 ml) and the chloroform layer separated off, and friedwith sodium sulphate.

Filtration and evaporation yield a light brown semi-solid organo-solvproduct (0.102 g, 34% by weight). Analysis of this product bychromatography showed that the major products, corresponding to 26%yeild based on a lignin formula of (C₁₀ H₁₃ O₄)_(n), were:

    ______________________________________                                        Products        Relative Yield (%)                                            ______________________________________                                        Phenol          3                                                             P(OH)benzaldehyde                                                                             42                                                            Vanillie acid   5                                                             P(OH)acetaphenone                                                                             12                                                            Vanillin        28                                                            Aceto vanillone 5                                                             Syringaldehyde  4                                                             ______________________________________                                    

EXAMPLE 3

Phenol extracted straw lignin (obtained from Battelle) (0.260 g) waselectrolysed and worked up following the procedure described in Example2 above. A crude light orange mixture (0.073 g, 28% by weight) wasobtained and analysed by chromatography to show that the major productswere:

    ______________________________________                                        Products        Relative Yield %                                              ______________________________________                                        Phenol          4                                                             P(OH)benzoic acid                                                                             4                                                             P(OH)benzaldehyde                                                                             39                                                            Vanillic acid   9                                                             P(OH)acetophenone                                                                             9                                                             Vanillin        21                                                            Syringic acid   2                                                             Acetovanillone  3                                                             Syringaldehyde  9                                                             ______________________________________                                    

EXAMPLE 4

Organosolv spruce lignin (0.40 g) was electrolysed following theprocedure of Example 2, but with a nickel anode and nickel cathode. Ayellow semi-solid crude material (0.050 g, 13% by weight) was obtained.Chromatographic analysis of the material showed:

    ______________________________________                                        Products      Relative Yield (%)                                              ______________________________________                                        Phenol         4                                                              Vanillic acid Traces                                                          Acetovanillone                                                                              14                                                              Vanillin      80                                                              Syringaldehyde                                                                              Traces                                                          ______________________________________                                    

corresponding to about 14% overall yield.

EXMAPLE 5

Organosolv Bagasse (0.100 g ) was electrolysed using the proceduredescribed in Example 2. A light orange solid (0.028 g, 28% by weight)was obtained. Analysis of this by chromatography showed the followingproduct distribution

    ______________________________________                                        Product         Relative Yield (%)                                            ______________________________________                                        Phenol          37                                                            P(OH)benzoic acid                                                                             8                                                             Vanillic acid   4                                                             Syringic acid   Traces                                                        P(OH)benzaldehyde                                                                             7                                                             Vanillin        9                                                             P(OH)acetophenone                                                                             8                                                             acetovanillone  6                                                             Syringaldehyde  5                                                             ______________________________________                                    

Corresponding to 26% overall yield.

EXAMPLE 6

Kraft aspen lignin (0.40 g) was electrolysed following the procedure ofExample 2, but with a nickel anode and nickel cathode. A light orangesolid material (0.040 g, 10% by weight) was obtained which onchromatographic analysis showed the following product distribution:

    ______________________________________                                        Product         Relative Yield (%)                                            ______________________________________                                        Syringic acid   Traces                                                        P(OH)benzoic acid                                                                             Traces                                                        Vanillin        49                                                            Acetovanillone  12                                                            Syringaldehyde  40                                                            ______________________________________                                    

REFERENCES

1. V. D. Davydov et al `Tezisy Dokl. Vses. Konf. Khim. Ispolz Lignina`6th 1975 (pub 1976) pp 122-5 (USSR).

2. E. I. Kovalenko et al `Tr. Novocherk Politeckh Inst` 320 69-73

3. E. I. Kovalenko et al `Zh. Prikl. Khim` 50 (8) 1741-1744 (1977).

4. L. V. Bronov et al `Khim Drev` (1) 40-44 (1976).

We claim:
 1. A process for the cleavage of lignin at a yield greaterthan 6% wherein by passage of an electric current through an aqueousalkaline solution of the lignin at a temperature above 100° C. whilstmaintaining mixing of the soluton the lignin is cleaved into smallermolecules than the starting lignin.
 2. A process as claimed in claim 1characterised in that the alkali is an alkali metal hydroxide.
 3. Aprocess as claimed in claim 2 characterised in that the concentration ofalkali is 2.5-3.5M.
 4. A process as claimed in claim 1 characterised inthat the temperature is 170°-190° C.
 5. A process as claimed in claim 1characterised in that the current density is 0.2-10 mAcm⁻².
 6. A processas claimed in claim 5 characterised in that the current density is 3-5mAcm⁻².
 7. A process as claimed in claim 1 characterised in that thecathode is selected from the group consisting of nickel, copper,vitreous carbon and lead and the anode is selected from the groupconsisting of copper, vitreous carbon and nickel.
 8. A process asclaimed in claim 7 characterised in that both the cathode and the anodeare made of nickel.
 9. A process as claimed in claim 1 characterised inthat the lignin is subjected to prehydrolysis prior to passage of theelectric current.
 10. A process as claimed in claim 9 characterised inthat the prehydrolysis is carried out above 100° C. using an aqueousalkali metal hydroxide.
 11. A process as claimed in claim 10characterised in that pre-hydrolysis is carried out at 170°-180° C. 12.A process as claimed in claim 1 applied to spruce, straw, organosolv,bagasse, aspen or HF lignin or lignin derived from wood pulp processing.